Many processes require controlled, variable rate feeding of fine particle solids into various process units. Many methods of accomplishing this are well known, but all exhibit substantial negative aspects.
Feeders for fine solids can generally be divided into those that feed the material in the deaerated state and those that require aeration to aid material flow. Some examples of deaerated fine solids feed devices are vibrating feeders, variable speed belt feeders, variable speed screw feeders, and variable speed screw feeders with flow enhancing features, such as conditioning mixers or augers.
Unfortunately, there are undesirable characteristics associated with each of these types of existing feeders. For example, vibrating feeders have poor control of feed rate and are prone to flooding with fluidizable solids resulting in uncontrollable flow through the feeder. Variable speed belt feeders are expensive and also are prone to flooding with fluidizable solids. Variable speed screw feeders sometimes have erratic mass flows as the density of the solids changes depending on feed hopper design, are prone to flooding with fluidizable solids, and are prone to leakage through shaft seals. Variable speed screw feeders with flow enhancing features are expensive and often require very tall “mass flow” hoppers to maintain constant speed vs. mass flow characteristics. Additionally, all of the feeders are prone to leakage through shaft seals.
Aeration of fine solids, for example by use of air slides or other fluidizing devices in the hopper above the fine solids feed device, can produce much more uniform solids densities so that volumetric feeders can provide more constant mass flow rates. The only widely used example of a fine solids feed device requiring aeration of the solids is the variable speed rotary feeder or “star” feeder. These devices produce good flow control and are reliable.
Unfortunately, these variable speed rotary feeders are expensive and have poor wear characteristics resulting in frequent need for repair or replacement. In addition, as these rotary feeders wear, the close tolerances that give them good feed rate control are lost. As these close tolerance fits widen with wear, solids leak through them at an increasing rate. Because of this, the variable speed controls must turn the rotary feeders ever more slowly to maintain a constant feed rate as wear increases. Often, worn rotary feeders cannot maintain the feed rate accuracy needed for the process being fed. In addition, rotary feeders have shaft seals that often leak solids to the surrounding environment as wear increases.